1. Field of the Invention
This invention relates to display devices. In particular, the invention relates to a structure for maintaining the state of a display device using an induced electric field.
2. Related Art
Flat-Panel Displays (FPDs), which are commonly used in laptop computers and other portable applications require low power consumption and compact size, in addition to good visual display characteristics. One class of technology used for FPDs involves the manipulation of particles that are suspended in a liquid medium to alter the reflective and transmissive properties of the medium.
There are two general types of particle displays, the first is often referred to as a suspended particle display (SPD). In SPDs the orientation of the particles is selectively controlled to produce the optical contrast required for a display. In an electrophoretic image display (EPID) the distribution of a particle population is selectively controlled in order to produce the optical contrast required for a display. In both cases an electric field is used to control the particles. It should be noted that particles in both display types are suspended in a liquid medium, and in one case the response to the electric field is with respect to orientation, and in the other with respect to distribution.
SPDs are attractive due to their wide viewing angle, high optical transmission and ease of fabrication. With SPDs, light valve action is obtained when sub-micron sized particles with an asymmetric, plate-like shape align with an externally-applied electric field, and thus permit light to pass through (the xe2x80x9clightxe2x80x9d state). This alignment occurs because the external field induces a dipole moment in the molecules of the particles. In the absence of the external field, the particles orient randomly due to brownian motion, and consequently block light (the xe2x80x9cdarkxe2x80x9d state).
For example, crystals of iodoquinine sulfate or related compounds may be dispersed in an organic liquid, and since the crystals are dichroic, there is a large difference between the absorption in the unaligned state in comparison to the aligned state. In the absence of an applied field, the random orientation produces a bluish-black appearance, and in the aligned state there is very little absorption and good contrast can be obtained with a white background. A significant disadvantage of SPDs is that the light areas of the display must be continuously energized with the external electric field to maintain the display, thus consuming energy even when the image on the display is static. SPDs also typically lack a clear voltage threshold, and require active-matrix addressing for high resolution.
In EPIDs, the particles used in the display are electrically charged and may either have a color that contrasts with the liquid used to suspend them, for example white particles in a dark blue dye, or may be divided into particles of two contrasting colors with opposite charge. The particles migrate under the influence of an applied electric field to the front or back of the display, producing a light or dark region when viewed from the front. The EPID operates by reflection and absorption as opposed to transmission. Although EPIDs have inherent memory, there is no voltage threshold, making multiplexed displays difficult.
Thus, the need exists for a particle display that is able to retain a displayed image without an applied external field. There is also a need for a particle display that has a voltage threshold that enables multiplexing.
A particle display device having a monolayer of a bistable polarizable molecule on either or both sides of a gap filled with a liquid containing suspended particles is disclosed. The monolayers and the liquid filled gap are disposed between patterned electrodes that are used to apply an electric field across the layers of the gap, thus influencing the orientation or the distribution of the suspended particles. The optical transmission or reflectance of the gap is dependent upon the applied electric field. The polarizable monolayers respond to the applied electric field by assuming one of two stable polarized states. The field provided by the dipoles of the molecules in the monolayer remains after the external field is removed, thereby sustaining the electric field influence on the suspended particles of the display device.
In one embodiment of the present invention, an SPD having plate-like or tabular particles suspended in a liquid filled gap between two transparent substrates has a monolayer of a bistable polarizable molecule applied to the surface of each of the substrates. The bistable polarizable molecule has two stable states separated by an electric field threshold, with one state having a higher electric dipole moment than the other, or a reversed dipole. When an external field of sufficient strength is applied across the substrates, the dipoles on opposite sides of the gap are in opposite complementary states and provide an internal field for the device that remains after the external field is removed.